A standard orthopedic procedure, for instance to splint the lateral portion of the distal tibia, entails reducing the fracture, drilling at least one hole to each side of the fracture, and then securing a plate across the fracture with screws. Similarly it is known, for example to fix a fragment of a broken medial malleolus, to drill a hole through the fragment into the bone and secure the fragment in place with a screw. The type of screw used depends on the type of bone tissue it will have to hold in. Cortical screws are intended to hold at the hard outer cortex of the bone while cancellous screws hold in the soft interior of the bone.
Thus a cancellous screw has a very deep screwthread intended to bite into the spongy interior of the bone. Unfortunately, once the fracture has knitted, this type of screw is fairly difficult to remove due to the adhesion of the spongy bone to its considerable surface area and the normally good adherence of bone to metal. In addition the hold of such a screw is frequently poor since the material it is holding in has little elasticity. Thus when stressed the screw either holds solidly or comes completely loose. The screw can loosen or pull out if stressed beyond a predetermined relatively low limit, there being no elasticity to the connection.
Accordingly a bone fastener is known that uses a synthetic-resin sleeve that is set into a hole bored in the bone, and then the screw is threaded into this sleeve. The result is a somewhat elastic connection, but the hold of the sleeve is often poor, at least until the screw is installed in it. In addition use of such a system is often somewhat cumbersome.